This invention relates to multi-wavelength optical signal amplification devices and optical telecommunications systems utilizing such devices, and in particular to a dual amplification path (multichannel) optical amplifier having selectively located spectral filtering for suppressing crosstalk induced multipath interference (MPI), return loss (RL), and self oscillation, and which also provides for a desired level of noise figure performance and output power performance (i.e., pump utilization efficiency) notwithstanding increased filter insertion loss.
The term "crosstalk" as used hereinafter will refer to an amplifier gain and reflection dependent phenomenon that is the genesis of multipath interference (MPI), return loss (RL), and self-oscillation, each of which are detrimental to good performance of a fiber optical communications system including an optical amplifier. The source of crosstalk induced MPI is illustrated in FIG. 1, which very generically shows a bi-directional optical amplifier including a west-to-east optical signal transmission/amplification path 1-2-3 (where 1 and 3 are a west and east reflection, respectively, and 2 is a gain from west-to-east) for wavelengths .DELTA..lambda..sub.1, for example, and an east-to-west optical signal transmission/amplification path 3-4-1 (where 4 is a gain from east-to-west) for wavelengths .DELTA..lambda..sub.2. A .DELTA..lambda..sub.1 MPI loop is represented by nodes 1-2-3-4-1 (i.e., .DELTA..lambda..sub.1 input-G.sub.1 -R.sub.E -G.sub.2 -R.sub.w). Interference between originally transmitted .DELTA..lambda..sub.1 signals and .DELTA..lambda..sub.1 signals traversing the MPI loop gives rise to MPI. Likewise, node path 3-4-1-2-3 represents a .DELTA..lambda..sub.2 MPI loop.
Return loss (RL) refers to .DELTA..lambda..sub.1 signals traversing nodal path 2-3-4 (i.e., G.sub.1 -R.sub.E -G.sub.2), and/or .DELTA..lambda..sub.2 signals traversing path 4-1-2, and represents the effective return reflectivity of the amplifier as seen by the communication system.
Self oscillation in the amplifier (laser oscillation) will occur when a loop or cavity is set up in which the gain exceeds the losses. Therefore, e.g., if G.sub.1 +R.sub.w +G.sub.2 +R.sub.E &gt;0, then lasing will likely occur.
Although the following specification will describe the invention in terms of a bi-directional (two counter-directional amplification paths) optical amplifier, the invention equally applies to a multichannel, unidirectional (two co-directional amplification paths) optical amplifier.
A bi-directional optical signal amplifying device may typically provide a signal amplification path in substantially one direction (e.g., east to west) for one or more in-band communication channels within a particular frequency band (e.g., the "red" band or hereinafter, .DELTA..lambda..sub.1), and a second signal amplifying path in a counter propagating direction (i.e., west to east) for one or more in-band communication channels in a different frequency band (e.g., "blue" band or hereinafter, .DELTA..lambda..sub.2). Optical amplifiers used in optical communication transmission systems typically incorporate an optical isolator in the amplification path for filtering unwanted reflections or to suppress the build up of spontaneous emission, the effects of which impair amplifier and system performance. Although it is well known that most "all optical" amplifiers, such as erbium doped fiber amplifiers (EDFA's) and semiconductor amplifiers, for example, will amplify an input signal regardless of the direction that the signal enters the device, the use of an isolator in the amplification path essentially restricts such a device to substantially unidirectional operation. Optical amplifiers that are functionally bi-directional, on the other hand, and particularly those that include a substantially uni-directional amplification path for each counter propagating signal band, respectively, require means for primary signal routing through the respective counter directional amplification paths. The means for routing the primary counter propagating signals may include, for example, optical circulators or wavelength selective directional filters at each input/output port of the bi-directional amplifier. Optical circulators are not preferred primary signal routing components for use at the input/output of a bi-directional optical amplifier because they are not wavelength selective devices and they are expensive. Currently available wavelength selective directional filters, particularly single stage components, lack the capability to provide the desired degree of spectral band discrimination within a desired narrow spectral range. For instance, in an EDFA, the gain spectrum window is on the order of 30 nm (1530-1560 nm). As shown in FIG. 1, a typical interference filter can provide approximately 10 dB spectral band discrimination through attenuation from reflection. This occurs, however, over a finite spectral range accompanied by a spectral "dead zone" of about 3-10 nm adjacent the signal band, instead of ideally as a step function, as shown in FIG. 5. The dead zone thus reduces the communication signal channel availability in an already limited spectral window. Moreover, the 10 dB attenuation typically is not sufficient to eliminate MPI, RL and self oscillation effects due to, for example, reflected and double reflected .DELTA..lambda..sub.2 light (from connectors or Rayleigh scattering) propagating in and being amplified by the primary amplification path for .DELTA..lambda..sub.1 light, and vice-versa. More specifically, we have found that the most critical need for wavelength selective isolation in a bi-directional optical amplifying device is to suppress crosstalk induced MPI, RL, and self oscillation. Even with the use of isolators in the uni-directional amplification paths, MPI, for example, can occur due to light (e.g., in-band .DELTA..lambda..sub.1) that propagates through the amplifier, gets reflected by some mechanism in an optical path of the system, and counter propagates through the amplifier along the primary amplification path for .DELTA..lambda..sub.2 light (i.e., as out-of-band .DELTA..lambda..sub.1, by going through the 10 dB wavelength selective routing filter via the nominally suppressed path for .DELTA..lambda..sub.1), hitting another system reflection on the other side of the amplifier, and finally propagating again in the original intended direction of .DELTA..lambda..sub.1 (as in-band .DELTA..lambda..sub.1), to be reamplified. One proposed solution to this problem is to increase the spectral isolation at the amplifier input/output routing locations to the primary, substantially unidirectional amplification paths for the respective in-band signals by, for example, using multi-stage filters at the input/output ports of the amplifier. This, however, also introduces increased insertion loss into the device which is not preferable as it is well known to those skilled in the art that increasing the insertion loss at the input end of an optical amplifier results in an overall increase in noise figure (due to signal spontaneous beat noise) of the device, while increasing the insertion loss at the output end of the amplifier results in decreased output power for a given pump power.
The inventors have therefore recognized a need for providing means for efficiently routing the respective communication signal bands into and out of the amplifier and, moreover, for suppressing unwanted (out-of-band) wavelength propagation through the amplifier that, if unsuppressed, results in crosstalk induced MPI, RL, and self oscillation, while not adversely impacting the noise figure and output power performance of the amplifier due to the increased insertion loss resulting from the spectral filtering.